This MergePolicy is used for upgrading all existing segments of
an index when calling IndexWriter.forceMerge(int).
All other methods delegate to the base MergePolicy given to the constructor.
This allows for an as-cheap-as possible upgrade of an older index by only upgrading segments that
are created by previous Lucene versions. forceMerge does no longer really merge;
it is just used to "forceMerge" older segment versions away.

Warning: This merge policy may reorder documents if the index was partially
upgraded before calling forceMerge (e.g., documents were added). If your application relies
on "monotonicity" of doc IDs (which means that the order in which the documents
were added to the index is preserved), do a forceMerge(1) instead. Please note, the
delegate MergePolicy may also reorder documents.

Method Detail

shouldUpgradeSegment

Returns if the given segment should be upgraded. The default implementation
will return !Constants.LUCENE_MAIN_VERSION.equals(si.getVersion()),
so all segments created with a different version number than this Lucene version will
get upgraded.

setIndexWriter

Sets the IndexWriter to use by this merge policy. This method is
allowed to be called only once, and is usually set by IndexWriter. If it is
called more than once, SetOnce.AlreadySetException is thrown.

findMerges

Determine what set of merge operations are now necessary on the index.
IndexWriter calls this whenever there is a change to the segments.
This call is always synchronized on the IndexWriter instance so
only one thread at a time will call this method.

Determine what set of merge operations is necessary in
order to merge to <= the specified segment count. IndexWriter calls this when its
IndexWriter.forceMerge(int) method is called. This call is always
synchronized on the IndexWriter instance so only one thread at a
time will call this method.

maxSegmentCount - requested maximum number of segments in the index (currently this
is always 1)

segmentsToMerge - contains the specific SegmentInfo instances that must be merged
away. This may be a subset of all
SegmentInfos. If the value is True for a
given SegmentInfo, that means this segment was
an original segment present in the
to-be-merged index; else, it was a segment
produced by a cascaded merge.